Cooling of modern electronics is becoming a major technical challenge due to the advancements in the design of faster and smaller electric components. As a result, different cooling technologies are being developed to effectively remove heat from these components. The use of liquid coolants has become an attractive alternative to air due to their material densities and high heat transfer coefficients which allows the removal of more heat. Coolants can be used in both single phase and two-phase formats.
Liquid submersion technology (LST) is one of the most promising and innovative methods for cooling computers such as desktop and server computers. LST utilizes dielectric liquid as a coolant medium instead of air.
Efficient cooling of electronics can help extend their operational lifetime. Keeping electronics at low temperatures allows operation at a higher speed (overclocking of CPU for example) since it is easier to remove the extra generated heat in the circuit by their contact with the flowing coolants. Moreover, heat extracted from electronic equipment in large data centers using LST can be recycled for later use in other heating applications, thus reducing their operational cost and negative impact on the environment. The capacity for recovering more heat by the LST technique could also be augmented by further increase of the thermal conductivity of fluids.
One of the promising methods of enhancing thermal conductivity of a fluid is to disperse into the fluid nanomaterial made of substances of relatively high thermal conductivities. Based on the predictions of the Mean Field Theory, one would expect the thermal conductivity of the new hybrid fluid to be higher than the base fluid alone.
Coolants of various types are used in equipment and in manufacturing processes to remove waste heat with water being the most efficient element due to its high thermal conductivity and heat capacity. In many applications water is not suitable and hence oil is used instead. Various types of natural or synthesized oils are used such as soy oil, mineral oil, polyalphaolefin, ester synthetic oil, and synthetic fluorinated oil. The value of the thermal conductivity of these oils is between 0.1-0.17 W/m-K at room temperature which is much lower than the 0.61 W/m-K of water.
Carbon nanotubes are a known thermally conductive material. Carbon nanotubes are macromolecules of the shape of a long thin cylinder and thus with high aspect ratio. There are two main types of carbon nanotubes: single-walled nanotubes (“SWNT”) and multi-walled nanotubes (“MWNT”). The structure of a single-walled carbon nanotube can be described as a single graphene sheet rolled into a seamless cylinder whose ends are either open or capped by either half fullerenes or more complex structures including pentagons. Multi-walled carbon nanotubes comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk with a typical distance of approximately 0.34 nm between layers.
Basic research over the past decade has shown that carbon nanotubes could have a thermal conductivity of an order of magnitude higher than copper—3,000 W/m-K for MWNT and 6,000 W/m-K for SWNT. Therefore, the thermal conductivities of nanofluids containing nanoparticles is expected to be significantly higher than the conventional fluids alone. Experimental results have demonstrated that a carbon nanotube suspension showed the highest thermal conductivity enhancement—a 150% increase in conductivity of oil at about 1% by volume of multi-walled carbon nanotubes (Choi, et al., App. Phys. Lett, 2001, 79(14), 2252).
Despite the extraordinary thermal properties of carbon nanotube suspensions, it is not easy to produce a nanoparticle suspension with a sustainable stability and consistent thermal properties. Due to the hydrophobic nature of graphitic structures, carbon nanotubes are not soluble in any known solvent. They also have a very high tendency to form aggregates and extended structures of linked nanoparticles, thus leading to phase separation, poor dispersion within a matrix, and poor adhesion to the host. However, stability of the nanoparticle suspension is important for practical industrial applications. Otherwise, the thermal properties of a nanofluid, such as thermal conductivity, will constantly change as the solid particles gradually separate from the fluid.
The superior thermal conductivity of carbon nanotubes and their derivatives has been recognized for a long time, but their use in cooling of electronics has not been extensively tested due to the high electric conductivity of carbon. One reason for this is the potential interaction of carbon clusters with electric circuits if their concentrations reach critical level. This condition can be avoided if the concentration of the nanomaterial is kept well below the percolation threshold. Additionally, one might expect that the dielectric breakdown voltage of the fluid will become smaller with the addition of the nanomaterial thus leading to circuit breakdown.